The invention relates to a voltage regulator for converting an input voltage, which may be affected by a ripple, into an output voltage which is substantially not affected by a ripple, comprising an input terminal for receiving the input voltage, an output terminal for supplying the output voltage in response to the input voltage, and current limiting means for limiting the maximum absolute value of an output current supplied from the output terminal.
Such a voltage regulator is known from Japanese patent abstract JP 2-136029 A. The known voltage regulator comprises a current mirror with an input and an output and, a bipolar transistor whose base is connected to the current mirror and whose emitter forms the output terminal of the voltage regulator. The known voltage regulator further comprises a voltage divider which consists of two resistors connected in series. The voltage divider is connected between the emitter of the bipolar transistor and a supply voltage terminal. The known voltage regulator further comprises a comparator, a first current source which supplies a comparatively small current, and a second current source which supplies a comparatively large current. A switch is connected in series with the second current source. The comparator is connected by a first input to the common junction point of the two resistors connected in series, and is connected by a second input to a reference voltage source, and is connected by an output to a control electrode of the switch. In a normal operational state of the voltage regulator, the switch is in the conducting state. The current supplied to the input of the current mirror in that case is determined by the sum of the currents supplied by the first and the second current source. This current is delivered from the output of the current mirror to the base of the bipolar transistor. The bipolar transistor amplifies this current and delivers the amplified current to the voltage divider. As the current through the voltage divider rises, the voltage at the first input of the comparator will become greater than the voltage at the second input of the comparator at a given moment. As a result of this, the voltage at the output of the comparator changes, such that the switch switches from the conducting state to a non-conducting state. In this state, the current supplied to the input of the current mirror is dependent on the first current source only. As a result, the current supplied from the output of the current mirror is reduced, so that the current supplied from the emitter of the bipolar transistor to the voltage divider is limited.
A disadvantage of the known voltage regulator is that the current limitation is achieved in a comparatively complicated manner.
It is an object of the invention to provide a voltage regulator which reduces the above mentioned disadvantage.
According to the invention, the voltage regulator mentioned in the opening paragraph is for this purpose characterized in that the current limiting means comprise a current limiting transistor with a main current path, and in that the current limiting means are designed such that, if the voltage across the main current path is higher than a given threshold voltage of the current limiting transistor, at which the current limiting transistor acts as a current source, the maximum absolute value of the output current is limited.
The invention is based on the recognition that the transistor is in its linear operational range as long as a voltage across the main current path of a transistor lies below a certain limit, so that the transistor behaves as a resistor, and on the recognition that, as the voltage across the main current path rises, there comes a moment when the voltage across the main current path exceeds said limit, so that the transistor starts behaving as a current source. The transistor thus acts as a current limiting transistor. The current limiting transistor may be constructed, for example, with a field effect transistor. When the drain-source voltage of the field effect transistor is smaller than the difference between the gate-source voltage and the so-called threshold voltage Vt, the field effect transistor is in its linear operational range. When the drain-source voltage of the field effect transistor is higher than the difference between the gate-source voltage and the so-called threshold voltage Vt, the field effect transistor is in its saturation range, wherein the field effect transistor acts as a constant-current source. The current limiting transistor may alternatively be constructed with a bipolar transistor. When the collector-emitter voltage of the bipolar transistor is below the so-called saturation voltage, the transistor is in saturation and behaves more or less as a resistor. When the collector-emitter voltage of the bipolar transistor is greater than the so-called saturation voltage, the bipolar transistor is not in the saturated state. The bipolar transistor then acts as a constant-current source.
Further advantageous embodiments of the invention are defined in claims 2 and 3.